I think the problem is two-fold:
1. No one has quite gotten social networking working quite right. MySpace and Facebook (and Linked-In) have come very close for their demographic, but they haven't broken out past those demographics.
2. As with many of these such things, the prospect of monetization draws people to create clone sites in the hopes of future riches.
I'm not saying that either of these things is bad, I kind of think of it as "The American Way" - in the freedom to succeed sense, not the jingoistic sense. Entrepreneurship and hard-working small-business people at their finest!
To ask a different question that many people feel very strongly about: How many wikis do we need?
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They're Dreamhost coupons!